1. Field of the Invention
The present invention relates to a polycarbonate resin composition for use in the production of a substrate for an optical information medium. More particularly, the present invention is concerned with a polycarbonate resin composition for use in the production of a substrate for an optical information medium, comprising (A) an aromatic polycarbonate resin having a weight average molecular weight of from 13,000 to 18,000, wherein the aromatic polycarbonate resin (A) is produced by subjecting an aromatic dihydroxy compound and a carbonic diester to a transesterification reaction, and is substantially free of a chlorine atom, and (B) a partial ester obtained from a saturated aliphatic carboxylic acid having 10 to 30 carbon atoms and a dito hexahydric alcohol, the partial ester (B) having an acid value of from 2 to 20 mgKOH. Use of the polycarbonate resin composition of the present invention in the production of a substrate for an optical information medium is advantageous in that a substrate (for an optical information medium) which has high mechanical strength and in which the occurrence of a “cloud” (i.e., haze) is suppressed, can be produced by the so-called “high-cycle molding” (i.e., the molding can be performed with a short cycle time). Therefore, the polycarbonate resin composition of the present invention can be very advantageously used in the production of a substrate for an optical information medium, such as an optical disc (e.g., a CD or a DVD).
2. Prior Art
Polycarbonates have been widely used in various fields as engineering plastics having excellent properties with respect to heat resistance, impact resistance and transparency. Especially, due to the recent rapid progress of information technology, there has been a growing demand for polycarbonates for producing storage media for music and images, and storage media for digital information (such as a storage medium for a personal computer). Nowadays, polycarbonates have become indispensable resins for producing optical discs and optical cards, such as a CD, a CD-ROM, a CD-R, a DVD-ROM and a DVD-R.
In the production of a substrate for an optical information medium, such as an optical disc, formation of precise microgrooves and micropits on the substrate is necessary. Polycarbonates for use in the production of such a substrate need to have high transferability and excellent optical properties, such as low birefringence. Therefore, low molecular weight polycarbonates having a weight average molecular weight of about 15,500 and having a high melt fluidity have hitherto been used in the production of the substrates for the optical information media.
In the production of the substrates for the optical information media, polycarbonates produced by the phosgene process have conventionally been used. However, the use of the polycarbonates produced by the phosgene process in the production of the substrates for the optical information media is accompanied by the following problems: (1) phosgene used in this process is toxic and thus difficult to handle; (2) methylene chloride used as a solvent in this process causes polycarbonates to contain impurities (chlorine ions and residual methylene chloride) which not only adversely affect the thermal stability of the polycarbonates during the molding, but also causes corrosion of a mold used for the molding of the polycarbonates; and the quality of the produced substrate for the optical information medium becomes poor under a moist heat environment. In order to alleviate these problems accompanying the phosgene process polycarbonates, various proposals for polycarbonate compositions containing decreased amounts of the above-mentioned impurities and methods for decreasing the amounts of the impurities contained in the polycarbonates have been made in a number of documents, including Unexamined Japanese Patent Application Laid-Open Specification No. Sho 63-316313 (corresponding to U.S. Pat. No. 4,880,896), Unexamined Japanese Patent Application Laid-Open Specification No. Hei 4-146922, and Unexamined Japanese Patent Application Laid-Open Specification No. Sho 63-97627 (corresponding to U.S. Pat. No. 4,798,767).
However, these methods pose problems in that the impurities (chlorine ions and residual methylene chloride) cannot be completely removed from polycarbonates and that the removal of impurities from polycarbonates requires a great deal of labor. Therefore, polycarbonates produced by the transesterification process, which uses no phosgene or methylene chloride, have recently been drawing attention.
However, the transesterification polycarbonates have the following problems. When a substrate for an optical information medium (hereinafter frequently referred to as a “disc”) is produced from a transesterification polycarbonate, the substrate produced exhibits poor properties, as compared to the properties of a substrate produced from a phosgene process polycarbonate. Specifically, when a disc is produced from a transesterification polycarbonate, problems occur not only in that the disc produced exhibits low mechanical strength, but also in that marked occurrence of a cloud (i.e., haze) is encountered during the molding, thus rendering it impossible to obtain an excellent substrate for an optical information medium. Further, the transesterification polycarbonates have a defect in that, when an attempt is made to shorten the time of molding cycle in order to improve the productivity of the disc (i.e., when a disc is produced by the “high-cycle molding”), the occurrence of a cloud is likely to be more vigorous. Therefore, it has been desired to improve the transesterification polycarbonates with respect to these properties.
It is well known that the mechanical strength of a disc can be improved by using a polycarbonate having an increased molecular weight. However, this method poses a problem in that the transferability of the polycarbonate with respect to the microgrooves and micropits on the disc becomes lowered or the birefringence of the disc is increased. Therefore, such method is not employed in the art.
On the other hand, with respect to a cloud (i.e., haze), it is considered that a cloud is caused by poor mold release of a disc in the molding process. Therefore, there have been made various proposals on mold release agents.
For example, in an attempt to improve the transferability of the polycarbonate, Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-113201 proposes a molded article for optical use, comprising a polycarbonate composition containing 0.01 to 0.2 part by weight, relative to 100 parts by weight of the polycarbonate, of a monoglyceride of a saturated aliphatic monoacid having 16 to 22 carbon atoms. It is considered that the monoglyceride used in this patent document is a commercially available product. In this connection, it should be noted that, as described below, the acid value of a commercially available monoglyceride of an aliphatic acid is approximately 1 mgKOH. Therefore, the acid value of the monoglyceride used in this patent document does not fall in the acid value range (of from 2 to 20 mgKOH) prescribed for the partial ester used in the present invention.
On the other hand, in an attempt to ensure good mold release properties of the polycarbonate and suppress the occurrence of hydrolysis of the polycarbonate, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 7-169092 proposes a substrate for an optical information medium, comprising a polycarbonate composition containing a mold release agent which is comprised of a C10-C30 aliphatic acid ester (e.g., glycerol monostearate) and which has a pH value of 7 or less, wherein the mold release agent is present in an amount of 0.5% by weight or less, based on the weight of the polycarbonate composition. In this patent document, it is preferred that the aliphatic acid ester used therein has a pH value of from 4.0 to 6.5. On the other hand, as described below, when the acid value of the partial ester used in the present invention (which is in the range of from 2 to 20 mgKOH) is expressed in terms of a pH value which is obtained by taking into consideration the operation conditions as described in this patent document, the pH value in the range of from about 1.75 to about 2.75 is obtained. Therefore, the acid value of the partial ester used in this patent document does not fall in the acid value range (of from 2 to 20 mgKOH) prescribed for the partial ester used in the present invention.
In an attempt to improve the mold release properties, thermal resistance and transferability of the polycarbonate, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 8-73724 proposes a composition comprising 100 parts by weight of an aromatic polycarbonate resin having a terminal hydroxyl content of from 2 to 40 mole % and a molecular weight distribution (Mw/Mn) of from 2.0 to 2.8 as measured by gel permeation chromatography, and 0.01 to 0.1 part by weight of a partial ester obtained from an aliphatic carboxylic acid and a polyhydric alcohol. It is considered that the partial ester used in this patent document is a commercially available product. In this connection, it should be noted that, as described below, the acid value of a commercially available partial ester is approximately 1 mgKOH. Therefore, the acid value of the partial ester used in this patent document does not fall in the acid value range (of from 2 to 20 mgKOH) prescribed for the partial ester used in the present invention.
These conventional techniques have not yet satisfactorily solved the above-mentioned problems accompanying the polycarbonates and, hence, it has been desired to improve the conventional techniques. That is, it has been desired to develop a molding material which comprises a polycarbonate resin produced by the transesterification process and which is advantageous not only in that it can be used in the production of an excellent substrate for an optical information medium, the substrate exhibiting high mechanical strength and suppression of occurrence of a cloud (i.e., haze), but also in that the molding material exhibits excellent suitability for high-cycle molding (i.e., excellent aptitude for being molded with a short cycle time).